Low-density foci are a manifestation of CT. CT images are represented in different shades of gray, reflecting the degree of absorption of X-rays by organs and tissues. Therefore, as with the black and white images shown on X-ray images, black shadows indicate areas of low absorption, i.e., low density, such as in the lungs, and white shadows indicate areas of high absorption, i.e., high density, such as in the bones. High or low density does not mean absolute benign or malignant, and should be judged based on the comparison of imaging between different organs at different times. If the low-density foci are suspected to be malignant, puncture with a fine needle can be performed to obtain pathology for examination if necessary, and pathology results such as tissue biopsy can also be performed. If the pathological result is benign, follow-up can be continued. If the pathology is malignant, radical surgery is required.